Energy recovery, as used herein, encompasses a variety of techniques by which energy is transferred from one subsystem of a larger system to another in order to minimize the amount of energy that must be input to the system for it to perform its function. Energy recovery systems are being developed to counter increasing energy costs and to reduce pollutants and greenhouse gasses. Certain of these energy recovery techniques are referred to as “regenerative,” meaning that energy is stored and then reapplied to do work. The most widespread example of this technology can be found in braking regeneration systems. These systems produce energy during braking in a way that can be readily stored, e.g., as electrical energy or hydraulic compression, as opposed to employing friction to brake, which generates heat that is usually just released into the brake's surroundings. The stored energy can be used to later supplement engine power, thereby effecting an improvement in overall fuel efficiency.
Recent advances in high pressure (6000-8000 psi), ruggedized, safe pneumatic components and subsystems have made pneumatic energy recovery a practical option, in many cases compressed air is used both as the energy storage medium and the working medium. Pneumatic energy recovery systems are generally smaller, lighter, and simpler than either of their electric or hydraulic counterparts.
A pneumatic energy recovery system, in its purest sense, has an optimized air compressor to store energy in the form of compressed air and a complementary air motor that operates on the expansion of air so as to utilize the stored energy in the compressed air to do work. The optimal configuration is a unified compressor and motor that can run in both directions, i.e., as a compressor and a motor, to implement thermodynamically reversible processes to the fullest extent practicable. Efforts to achieve an efficient thermodynamically reversible compressor/motor have been ongoing.